Power supply switching apparatus with overload detection, also referred to as a ‘smart power switch’, is used in many applications, including automotive equipment, especially to control vehicle lighting, for example. It is particularly important to protect automotive wire harness and printed circuit boards (PCB) during the turn-on of the power supply, as well as switch components of the power supply itself, even in the event of severe short-circuits. The overload detector responds to an overload by providing a fault signal that can be used to limit or avoid damage. The present invention is particularly, but not exclusively, usable in such applications. Power switches with overload detection are disclosed in Patent Applications PCT/EP2004/014895, PCT/EP2005/005208, PCT/EP2005/005211 and PCT/EP2005/005212.
Such smart power switches are subject to stringent power consumption limits during their quiescent operational mode and it is desirable to avoid powering and activating overload detection before the load is to be turned on. Conveniently, the overload detector or sensor is powered and activated by the same control signal as turns on the load. Certain conventional overload protection circuits use a sensor comprising a sense resistor or Field Effect Transistor (‘FET’) and have a significant settling time of a few hundred microseconds (μsecs) when they are turned on; this already poses some issues for timely protection and prolongation of the reaction time should be minimised.
Moreover, power supply apparatus of this kind may be arranged to provide a progressive build-up of output current during turn-on, known as round-shaping, which is implemented to meet Electro-Magnetic Compatibility (‘EMC’) requirements, notably to reduce radio frequency emissions. In order to achieve EMC requirements, the output slew-rate, that is to say the progressive turn-on of the output switch of the power supply, is conveniently made dependent on the output voltage (Vout), which enables the use of a passive filter component to be avoided, for example. The round-shaping can be achieved by sensing the output voltage to obtain a progressive variation of the control signal applied to the output switch, turning the output switch on progressively. However, this progressive turn-on also tends to prolong the overload detection reaction time.
The overload detector responds to the load impedance by providing a fault signal that can be used to limit or avoid damage. Overload detectors exist that function well to detect typical resistive overloads, that is to say where the load impedance still presents a significant resistance even if it is an order of magnitude less than the normal load, for example, at least after an acceptable initial time delay while the energisation of the detector itself builds up. However, in severe overload conditions, known overload detectors may not respond adequately to a severe short-circuit condition at the power supply output. For example, if the overload detector is responsive to Vout, a severe short-circuit at the load will pull Vout down and keep it relatively low during a prolonged turn-on time in spite of rapidly increasing power supply output current. The overload sensor will not be energised to detect an abnormal condition during the turn-on phase fast enough to prevent the power dissipation damaging the power supply output switch and/or also the load supplied with power (wire harness and/or PCB).
A need exists for power supply apparatus with rapid overload detection during the turn-on phase, especially against severe overload conditions.